The present invention relates to spread spectrum communication between two or more communication devices, and more particularly to time synchronization between such communication device.
Spread spectrum communication has been described as a means of communication in which the signal occupies a larger bandwidth than the minimum necessary to send the information. The spreading of the signal over the band is accomplished by means of a code which is independent of the communication information, and a synchronized reception with the code at the receiver is used for despreading and subsequent information recovery. Theory of Spread-Spectrum Communications--A Tutorial, R. L. Pickholtz, D. L. Schilling and L. R. Milstein, IEEE Trans. Commun., Vol. COM-30, pp. 855-884, May, 1982.
Networks of spread spectrum communications units employing frequency hopping may be employed for secure, anti-jam performance. In order for communication to occur between two (or more) such units, the operation of each unit must be timed synchronized to the operation of the other communicating unit(s).
The time synchronization has in the past been performed by one of several techniques. With one such technique, a synchronized unit transmits a synchronization signal that is long enough to allow an unsynchronized receiver to advance its internal time and compare the received signal to its internally generated synchronization signal. When the signals correlate, the unsynchronized receiver's internal time has been matched to that of the synchronized transmitter, and synchronization has been achieved.
In a second technique, a transmitted synchronization signal is received by an unsynchronized unit, which monitors simultaneously a number of frequencies thereby covering a span of time. The number of frequencies that must be monitored is dependent upon the time inaccuracy which must be resolved. The signal received on each frequency monitored is compared against an internally generated synchronization signal and the monitored frequency which achieves the proper correlation between its signal and the transmitted signal has the correct time.
With another technique, the network has a lower performance mode that is used for network entry. Under certain conditions, such as if heavy jamming is experienced, the network entry waveforms are repeated many times to ensure unsynchronized unit network entry.
With another approach, the synchronization waveforms are "hidden" within network communications. Unsynchronized units monitor network communications and synchronization is achieved afer a sufficient number of these "hidden" transmissions are received.
It would represent an advance over the art to provide a time synchronization technique for spread spectrum, frequency hopping communication devices which would allow unsynchronized units to request network entry, provide relatively short synchronization transmissions covering relatively large time uncertainties, employ one standard synchronization transmission supporting multiple network entry methods, and provide a synchronization waveform which can be made to look identical to typical network communication transmissions.
It would further be advantageous to provide a time synchronization technique for a network of frequency-hopping communication devices which does not require special network entry receiving or transmitting equipment.